Flooded Soils and Nitrogen Needs

Regardless of how quickly the crop returns to normal after an event like temporary flooding, questions will remain about how standing water might affect the amount of nitrogen left in the soil to meet the needs of the crop. Warm, saturated soils lose nitrogen (as gas back into the air) through the process of denitrification. We do not think that such losses have been very large in most fields, given the temperatures and the fact that most flooding was temporary. In better-drained fields, denitrification would be less, but percolating water has probably moved some of the nitrate-nitrogen deeper, perhaps below the root system or into tiles lines.

In central Illinois we have accumulated about 1,100 growing degree days (GDDs) since May 1, and about 930 GDDs since May 15. By the time corn accumulates 1,000 GDD, reaching about stage V13, it has accumulated about 20% of its dry weight and about 40% its season-long nitrogen accumulation (Abendroth et al., 2011) During this period the crop takes up 3-3.5 lbs. of nitrogen per acre per day, and by the time of pollination, it will have taken up about 60% of its nitrogen and produced about 40% of its dry weight.

At the time the crop reaches stage V13 (about head-high), it still has to take up 110 to 120 lbs. of nitrogen, and in years when June is wet, a common question is whether or not the crop might run out of nitrogen, leaving the crop short. While the need for 20 or more lb of N per week would seem to raise the possibility of a shortage, the production of plant-available N from soil organic matter through the process of mineralization is also at its maximum rate in mid-season.

For a crop with a good root system growing in a soil with 3% organic matter, mineralization at mid-season likely provides at least half the nitrogen needed by the crop on a daily basis. This means that normal amounts of fertilizer nitrogen, even if there has been some loss, should be adequate to supply the crop.

Though we could measure soil nitrogen present or apply urea by air on the wetter field or parts of fields where the crop shows deficiency, it would seem prudent to wait to see if the crop recovers its green color before going to this expense. The loss of crop color in wet soils is due mostly to loss of root function, and roots will need to recover before the canopy does. Even without adding more N, odds are good that the crop will recover and thrive in the coming weeks, providing the weather remains favorable.

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